In the field of semiconductor and LED manufacturing, vapor deposition processes such as Chemical vapor deposition (CVD), Metal-Organic Chemical Vapor Deposition and Atomic layer deposition (ALD) have played a critical role in depositing materials on substrates. All these processes require accurate delivery of precursor gas into reactor chamber in order to create a desired uniformity and quality of the layer on the substrate. Accurate delivery of such gas/vapor mixture requires precise measurement of the concentration of the precursor gas.
At present there is a wide variety of concentration sensors are available in market which use different sensing techniques such as acoustical technology, optical technology and mass spectroscopy. Among these technologies; the acoustical sensor such as PIEZOCON® gas concentration sensor (hereinafter “Piezocon Sensor”) sold by VEECO has demonstrated high accuracy and repeatable real time concentration measurement. It has been widely used and integrated in CVD and MOCVD tools for monitoring the precursor concentration and controlling the mass flow.
In the conventional Piezocon systems, a piezoelectric component is stacked on each side of the transducer, and mechanically coupled to a transformer. The interface between the piezoelectric components and the transformer is an epoxy that provides a mechanical bonding and also permits for the transmission of acoustic signal. However, despite the mechanical structure of the sensor being robust, it has been discovered that the piezoelectric components can separate from the transformer when the sensor is operated at a relatively high temperature (>100° C.). This is due to the coefficient of thermal expansion (CTE) mismatch between piezoelectric components and transformer, which causes mechanical deformation, stress and strain between components that result in mechanical bonding failures.
In addition to the mechanical challenge of the sensor at high temperature, it has been also discovered that the accuracy of measurement is degraded when the sensor is operated in a low pressure regime (below about 100 Torr). This is due to that the signal strength across the gap decreasing as the pressure of the gas mixture therein decreases, resulting in a small signal to noise ratio and therefore affecting the accuracy and reliability of measurement.
There is a need, therefore, for a sensor that can operates in low pressure and/or high temperature environments in which conventional systems either do not provide usable data or would be damaged.